remove theme_option

https://github.com/rust-lang/mdBook/pull/2795

theme/index.hbs

@@ -122,11 +122,11 @@
                             <i class="fa fa-paint-brush"></i>
                         </button>
                         <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
-                            <li role="none"><button role="menuitem" class="theme" id="all">{{ theme_option "全部" }}</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="tai">{{ theme_option "傣" }}</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="my">{{ theme_option "缅" }}</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="roman">{{ theme_option "罗马" }}</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="han">{{ theme_option "汉字注音" }}</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="all">全部</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="tai">傣</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="my">缅</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="roman">罗马</button></li>
+                            <li role="none"><button role="menuitem" class="theme" id="han">汉字注音</button></li>
                         </ul>
                         {{#if search_enabled}}
                         <button id="search-toggle" class="icon-button" type="button" title="搜索 快捷键：S" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">